1. Field of the Invention
The invention relates to a fluid operated linear actuator which provides infinitely variable linear positioning in two axial directions, particularly adapted for, but not limited to, use with servo mechanisms for controlling propeller pitch or power steering systems for marine vessels.
2. Prior Art
Double-acting positioning actuators have been known for a long time and commonly utilize an axially movable actuating rod slidably mounted within an elongated cylinder. Some of the actuators have one piston secured to the actuating rod, with two compression coil springs sandwiching the piston to center it within the cylinder. Later actuators utilized one compression coil spring mounted between two pistons, the pistons being rolling diaphragms having inner portions secured to the rod and outer portions secured to spaced positions on the cylinder wall. One example of the later type of actuator is shown in the present applicant's U.S. Pat. No. 3,838,630 issued 1 Oct. 1974 and entitled Double-Acting Positioning Linear Actuator. Although this actuator functions well in many applications, it has a relatively long overall length for the length of its stroke, thus limiting use of the actuator to applications where excessive overall length is not a major consideration. Furthermore, some of the earlier actuators, while being mechanically relatively simple, require manufacture and assembly of a multiplicity of parts, some of which were relatively costly to produce, and thus result in relatively high production and maintenance costs.